1. Field of the Invention
The present invention relates to an imaging optical system and an image reading apparatus using the same. The present invention is particularly preferable in reading a monochrome image or a color scanner, a digital copier, a facsimile, or the like, which uses an imaging optical element with various kinds of aberrations corrected in good balance, including a plurality of small off-axial reflecting surfaces having high resolution.
2. Related Background Art
Up to now, a flat-bed type image scanner has been proposed, for example, in Japanese Patent Application Laid-Open No. 3-113961 as an image reading apparatus (image scanner) for reading image information on an original surface.
The flat-bed type image scanner has an imaging lens and a line sensor fixed therein and moves only a reflection mirror, thereby subjecting the original surface to slit exposure scanning to read the image information.
In recent years, a carriage integral type scanning system has often been employed, which integrates a mirror, an imaging lens, a line sensor, and the like in order to realize simplification of an apparatus structure, and scans an original surface.
FIG. 9 is a schematic main part view of a conventional image reading apparatus of a carriage integral type scanning system. In FIG. 9, light flux irradiated from an illumination light source 1 directly illuminates an original 8 mounted on an original table glass 2, an optical path of a reflected light flux from the original 8 is folded inside a carriage 6 via first reflection mirror 3a, second, reflection mirror 3b, and third reflection mirror 3c in order, and the light flux is focused on a surface of a line sensor 5 by an imaging lens (imaging optical system) 4. Then, the carriage 6 is moved in a direction of an arrow A (sub-scanning direction) shown in FIG. 9 by a sub-scanning motor 7, whereby image information of the original 8 is read. The line sensor 5 in FIG. 9 is constituted by arranging a plurality of light receiving elements in a one-dimensional direction (main-scanning direction).
FIG. 10 is an explanatory view of a basic structure of the image reading apparatus of FIG. 9.
In the figure, reference numeral 4 denotes an imaging optical system; 5R, 5G, and 5B, line sensors for reading colors R (red), G (green), and B (blue), respectively, of the line sensor 5; and 8R, 8G, and 8B, reading areas on an original surface corresponding to the line sensors 5R, 5G, and 5B, respectively. The carriage 6 is scanning a stationary original surface in the image reading apparatus shown in FIG. 9. However, carriage scanning is equivalent to a state in which the line sensor 5 and the imaging lens 4 are stationary and the original surface 8 is moving as shown in FIG. 10. By scanning the original surface, an identical part can be read by the line sensors of different colors with a certain time interval. In the above-mentioned structure, in the case in which the imaging lens 4 consists of an ordinary refraction system, an axial chromatic aberration and a chromatic aberration of magnification occur. Thus, defocus or positional deviation occur in line images to be formed on the line sensors 5B and 5R as compared with the reference line sensor 5G. Therefore, when respective color images are superimposed to reproduce an original image, blur or deviation is conspicuous in a resulting image. That is, in the case in which performances of a high aperture ratio and a high resolution are required, the requirement cannot be met.
On the other hand, recently, it has been clarified that, even in a decentered optical system, it is possible to establish an optical system in which aberrations are corrected sufficiently by introducing the concept of a reference axis to make constituent surfaces thereof asymmetry and aspherical. For example, a designing method of the optical system is disclosed in Japanese Patent Application Laid-Open No. 9-5650 and examples of the design are disclosed in Japanese Patent Application Laid-Open Nos. 8-292371 and 8-292372.
Such a decentered optical system is called an off-axial optical system (an optical system which is, when a reference axis along a light beam passing through a center of an image and a center of a pupil is assumed, defined as an optical system including a curved surface whose surface normal line at a crossing point with a reference axis of a constituent surface is not on the reference axis (off-axial curved surface). In this case, the reference axis has a bent shape). With this off-axial optical system, since constituent surfaces thereof are generally decentered and eclipse never occurs even on a reflection surface, it is easy to establish an optical system which uses a reflection surface. In addition, the off-axial optical system has such characteristics that an optical path can be drawn around relatively freely and an integral type optical system is easily manufactured with a technique for integrally molding constituent surfaces.
On the other hand, an image reading apparatus such as a digital copier has not been constituted by an integral type optical system yet because a high resolution and a high speed are required for such an image reading apparatus. Since an imaging lens required in the image reading apparatus needs to be bright and have a high resolution, it is difficult to increase an angle of view in order to secure an optical performance. If the angle of view is small, the resultant optical path length increases.
On the other hand, in the case in which a color image is read, as the resolution of the imaging lens becomes higher, the optical performance is adversely affected by a difference of imaging positions for respective colors due to a chromatic aberration or by a chromatic aberration such as color shift in a screen.
On the other hand, it is required to increase brightness (Fno and transmissivity) of an imaging optical system as the reading speed of an image becomes high. However, since a reflectance on a mirror is poor compared with a transmissivity of a lens of a coated refraction system, in the case in which a large number of mirrors are used in order to fold a long optical path, light from an illuminated original cannot be guided to a line sensor efficiently.
On the other hand, in the case in which an integral type optical system is constituted by a technique for integrally molding off-axial constituent surfaces with glass or plastics, the accuracy of a surface or the tolerance of an interval between surfaces has to be made extremely strict in order to prevent deterioration of the performance due to a manufacturing error of each off-axial constituent surface, which causes increase in manufacturing costs.